Add JavaScript function to a dynamically created control

I tried all possible ways to add javascript code to my dynamically created control a date textbox but it is not working. I need to add multiple scripts like searchable dropdownlist and datepicker for date textbox. I am creating control on Page_Init and then attaching javascript on Page_Load but it is not working. Can somebody help, please? Thanks.

protected void Page_Init(object sender, EventArgs e)
{
    CreateControls();
}

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (fieldtype == "date")
            {
                string javaScriptString = @"function attachJSScript() {";
                javaScriptString += @"$('#<%= " + str + ".ClientID %>').datepicker({ dateFormat: 'dd/mm/yy' }).val();";
                javaScriptString += @"}";
                ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "attachJSScript", javaScriptString, true);
            }
        }
    }
}

When I view the page source, it shows

<script type="text/javascript">
//<![CDATA[
function attachJSScript() {$('#<%= txt_1_date_yes.ClientID %>').datepicker({ dateFormat: 'dd/mm/yy' }).val();}//]]>
</script>

I added this code for dropdownlist dynamically created control but it is not working either. Same code I used for the controls in aspx pages and that works.